Brave Lucina

Geirskögul (+Eff) +3 HP +16 Mt.
Effective against dragon foes. Grants Def+3. Grants Atk/Spd/Def/Res+3 to sword, lance, axe, bow, dagger, or beast allies within 2 spaces during combat, and if foe initiates combat against those allies, grants Special cooldown charge +1 per attack to those allies. (Only highest value applied. Does not stack.)

With her new fantastic refine, Brave Lucina becomes a fantastic support unit for non-magical allies. Possibly becoming one of the best support units in the game, due to her massive +3 buffs to all non-magical allies within 2 spaces as well as effectively giving them steady breath effect as well. While she has dragon effectiveness and great stats overall, Brave Lucina still wants to focus on being an arena support or just a support unit in general. Fury 4 is significant here as it allows Brave Lucina to hit high atk, spd & def stats which lets her soak Chill skills for her allies, while allowing her to be a monster in combat if ever needed. Chill skills in both B & seal allows her allies to deal with enemies easier and Close Guard provides even more buffs for her allies. The choice of Close Guard vs Distant Guard depends on your team comp, just like the rally skill she wants to take. While she can be running Link skills and movement skills, those only benefit one ally and can be tricky to use while double chill is consistent and benefits all her allies.

Boon: +Def (if at +0, her best bane in this build is -Res)
Weapon: Slaying Lance+ (+Spd)
Support: Reciprocal Aid
Special: Aether
Passive A: Steady Breath, Atk/Def Bond 3, Spd/Def Bond 3 (the Bond skills if she has an ally with Infantry Breath 3)
Passive B: Wrath 3 / Guard 3
Passive C: Atk or Spd Smoke 3, an Infantry skill, or a Wave or Tactic skill (preferably Wave unless she is used on a Mixed team)
Seal: Darting Stance 3 / Close Def 3 / Panic Ploy 3 / Quick Riposte 3
Summoner Support: Absolutely
Ally Support: Red unit for protecting her against Green units, or Green unit so she can protect them. Someone that can counter enemies like Hector or Grima would be great. If she has an ally with Infantry Breath (like Yarne), she can use a different A Passive (preferably a Bond skill to take advantage of the fact she needs to be adjacent to that ally to gain the Breath).

Super-sustain pseudo-Healer Brave!Lucina. With Slaying Lance+ (+Spd), this Brave!Lucina can get up to 39 Spd (43 at +10 in merges, then 45 at +10 dragonflowers), and due to the combination of it and her speed with Steady Breath, she can fully charge and activate an originally 5-cooldown skill like Aether in a single instance of battle. Reciprocal Aid is a great option for proccing Wrath. Obviously, her Special is Aether which is amazing at both damage and sustain. Activating this each time she battles an enemy is pretty brutal to a lot of enemies.
Her A skill is Steady Breath. While this does force her niche into becoming an Enemy Phase unit, a Def boon lets her reach up to 35 Def with its effect...and with a neutral base 34 Atk (48 with the weapon) she'll be retaliating pretty hard after tanking your opponent's attacks. Unfortunately, this Def increase does nothing against Res-targeting units (especially Green ones), so keep her away from them unless she's in a winning battle.
For her B skill, either Wrath 3 or Guard 3 are your best placements. Wrath 3 lets her charge Aether even faster and increases its damage by 10 (and therefore potential healing by 5), while Guard 3 gives her some safety against units that can deal some nice damage to her with their Specials.
Spd Smoke 3 is her safest option for her C skill, making it harder for consecutive opponents to double her (and making it easier for her to double them in return). Otherwise, her best options should be based on the team she's on.
As for her seal, Panic Ploy 3 is my favorite to use against Emblem teams. Darting Stance 3, however, is a lot better so as to increase her Spd. For a bit more Def/Res against her counterable enemies, Close Def 3 is equally if not more usable.
For your Ally Support, be sure to take an ally that can either help or be helped from Brave!Lucina's status as a lance unit. A strong unit that has lower (current) HP than she does will be able to be restored to full health from her Reciprocal Aid depending on her HP at the time, and she can make use of Steady Breath/Aether to get back up to full health for another pseudo-heal. A unit that can stand alongside her on the front lines, preferably one with a Distant Counter weapon/A Passive like Legendary!Ike, is also perfect for her (it also helps that he's a sword user with DC built into his weapon, so he can protect her against axe users like Arthur AND Tome users like Nino thanks to his natural Warding Breath).

S support with B!Roy, complements each other with weapon and movement type. Using pivot to increase movements or avoid situation.
S support with player.
[neutral - no boons nor banes]

A good defensive unit who can pair well with Brave Ike. She should be taking the hits for him and countering while he steps in to finish off those that Lucina isn't killing on her own. She should also be able to competently pair up with other Brave Units.